The present invention relates to an optical information medium such as an optical disk, an optical tape and an optical card. This optical information medium also includes a read only type optical disk such as a compact disc (CD), a laser disc (LD) and a digital video disc (DVD) and a write once type or an erasable type optical disk for a computer.
Heretofore, a method of densifying an optical information medium is equal to that of densifying a two-dimensional information medium. However, as the size of an information medium is limited for miniaturization of an apparatus and further, densification on a plane is limited because the size of a recordable mark is also limited due to a diffraction limit of light.
As a method of further densification, a method for reading out a three-dimensional record including depth is discussed. For example, Japanese published unexamined patent application No. Sho 59-127237 discloses a method of recording and reading out by focusing a beam on each recording layer in a disk provided with a multilayer recording layer. However, in an embodiment thereof, the concrete structure of an information medium is not shown and therefore, reliable recording according to the invention cannot be expected.
For example, Japanese patent Laid-Open No. Sho 60-202545 discloses the thickness of an information medium for forming a light spot at a diffraction limit in each layer of a disk provided with a multilayer recording layer. However, in an embodiment thereof, the structure of an information medium which can be fabricated at a low cost is also not shown clearly.
For example, Japanese patent Laid-Open No. Sho 54-130902 discloses a disk provided with a multilayer information layer. However, a signal to noise ratio (S/N) cannot be increased in this disk and planar recording density of each information layer cannot be also enhanced.
In an optical information medium, a reflective layer of a light beam is required to be provided to detect light reflected from the same face as an incident one of a light spot for recording and reading out. It is known formerly that metal with high reflectance such as aluminum and gold as metal used for such a reflective layer should be used and it is disclosed in for example on pages 12 and 84 of "Compact disc reader" written by Mr. Nakajima and Mr. Ogawa published on Nov. 25 in 1982 by Ohm.
Aluminum or an aluminum alloy is suitable of metals used for this reflective layer because its cost is low. However, if a conventional single thin film formed by aluminum or an aluminum alloy is used for a reflective layer, linear data density cannot be higher than some extent because noise is increased and S/N is not large enough and an optical information medium with large capacity cannot be provided. To enhance recording density of each layer two-dimensionally, an information reader which can irradiate a light spot for reading out with a shorter wavelength is used. However, a problem caused when an information medium with multilayer structure is applied to such an information reader is discussed in no conventional embodiment.
A first object of the present invention is to provide a multilayer optical information medium with planar high recording density wherein reading out of a three-dimensional record is enabled and which can be fabricated at a low cost.
A second object of the present invention is to provide a low-priced optical information medium or a low-priced multilayer optical information medium provided with a reflective layer which can enhance S/N and its linear data density sufficiently.
A third object of the present invention is to provide a multilayer optical information medium wherein the intensity of a read out signal is not deteriorated even if an information reader for a high recording density medium with a shorter wavelength of a light spot for reading out is used.